PROJECT SUMMARY/ABSTRACT Primary malignant brain tumor is associated with a high risk of family caregiver burden, which can negatively impact caregiver and patient well-being. Caregivers play an important role in caring for patients at home, but if caregivers become too overwhelmed to care for the patient, patient health care utilization can increase. There is strong evidence that high quality social support is a vital component to caregiver well-being, but barriers exist to caregivers receiving informal and formal support and few interventions for this population. Our objective is to address the caregiver social support gap by implementing a stepped-care intervention to support informal family caregivers (FCGs). We use a blended approach of technology and personal contact to leverage informal social support and connect caregivers to formal support services with the web-based electronic Social Network Assessment Program (eSNAP) and a Caregiver Navigator. We hypothesize that by intervening with the caregiver to address informal and formal support needs, we will improve caregiver and patient well-being and reduce patient health care utilization. Our Specific Aims are to: 1) Determine the efficacy of eSNAP + Caregiver Navigator support intervention on FCG well-being; 2) Determine the efficacy of eSNAP + Caregiver Navigator on patient well-being and health care utilization; and 3) Identify key intervention components using mixed methods to inform future intervention implementation. To achieve these aims, we will conduct a prospective, longitudinal randomized controlled trial of the 8-week eSNAP + Caregiver Navigator intervention. The intervention is targeted to family caregivers of patients with a primary diagnosis of malignant brain cancer, but both caregivers and patients will participate in the study. Participants will be recruited at the patient's first treatment planning appointment and randomly assigned to either the intervention condition or to a waitlist control condition. Caregivers in the intervention condition will receive access to eSNAP, a web-based app to help caregivers catalogue and visualize informal social network resources. eSNAP also provides a basic resource list. eSNAP will alert a trained Caregiver Navigator to contact participants who demonstrate a lack of support. The Caregiver Navigator will problem-solve with caregivers and make arrangements/referrals in ongoing phone consultations. The Caregiver Navigator's goal is to help caregivers identify and utilize informal and formal sources of support to place caregivers on a successful trajectory of support. Questionnaires will be collected from all participants at enrollment and 8 weeks, coinciding with patient clinic visits. Caregivers will also complete questionnaires at 4 weeks and a short debriefing interview at 8 weeks. Waitlist participants may then receive the intervention and will be asked to complete questionnaires mirroring intervention participants. Follow-up data will be collected at 6 months and 1 year. The proposed intervention supports caregivers to improve well-being and optimize health care utilization in the dyad, while also providing important foundational work in understudied FCGs and patients with primary malignant brain tumor.